Various methods of attaching a prosthesis to an amputee's residual limb have been utilized over the years. Despite advances in materials and design many of the older methods are still being used. The earliest methods were strap and belt type suspension systems. In this design, the amputee inserted the residual limb into the prosthesis and tightened a belt, typically made from leather, to hold the prosthesis to the limb. This design was fraught with practical difficulties, but for many years was the only design available. A second-generation design improvement uses flexible inner integral suction suspension sockets. These designs are especially useful for above-knee prostheses. The amputee simply pulls the prosthesis over the residual limb, while the flexible interior of the socket adheres to the limb creating a vacuum that suctions the prosthetic in place. A valve is installed within the socket to break the suction.
More recent designs have employed mechanical connectors, combined with a removable roll-on suction suspension sleeve. These mechanical connectors consist of: 1) lanyard systems utilizing a braided or twisted polymeric cord attached to the end of the suction suspension sleeve which passes through the socket where it can be secured external to the prosthesis; 2) shuttle lock systems, having a ring attached to the suspension sleeve to receive a pin inserted through the socket; 3) a ratchet type shuttle lock system, using a barbed plunger extending from the end of the suction suspension sleeve and engaging a ratchet mechanism on the prosthesis; and 4) gear driven clutch lock systems using a plunger with teeth to engage a gear mounted to a shaft turning on a one-way bearing where the plunger once engaged with the gear is restricted from upward vertical movement.
Most recently, smooth pin lock systems have been devised in an attempt to overcome the drawbacks of the earlier mechanical lock systems such as Gramnas U.S. Pat. No. 5,298,290 ('290). Gramnas uses a washer to lock the plunger. The washer is housed such that when the plunger pierces the interior of the washer the washer becomes biased against the plunger by springs, which flank the washer. The springs are actuated by a single bearing. The bearing serves to compress one spring while leaving the other to bias the washer against the interior of the housing. Thus the washer becomes angled and the plunger is prevented from exiting the lock. Because the Gramnas design does not rely on gears or ratchets to secure the sleeve to the socket less play is introduced into the lock. Despite the improvements that the smooth pin lock system offers, it has not been a complete solution and has introduced its own disadvantages.
All of the automatic locking mechanical connectors (ratcheting shuttle, gear driven clutch, and smooth pin) have the drawback of requiring the plunger and the lock to be aligned on the same axis. The nature of the design of the ratcheting shuttle and gear driven clutch type locks permits a certain amount of play or backlash that allows the residual limb to piston within the socket. The lanyard and shuttle lock systems do not automatically lock and cannot accommodate volume changes in the residual limb without manual adjustments. Volume changes occur as the amputee bears weight on the residual limb causing swelling or shrinkage in the fleshy portion of the residual limb. Limb shrinkage loosens the fit in the socket and causes the residual limb to slip deeper into the prosthesis. The practical result is that additional play is introduced requiring the amputee to make frequent adjustments to maintain a snug fitting. The ratcheting shuttle lock also suffers from this drawback because the discrete locking steps are spaced too widely to make fine adjustments.
Smooth pin systems must conform to tight tolerances in order to work properly. As components wear, the retention of the prosthesis becomes less secure. This is particularly true in smooth pin systems such as the Gramnas invention, where the smooth pin is restricted from moving out of the lock by a washer that contacts the plunger with a narrow and limited surface area. Over time, the limited surface area of the washer becomes worn, introducing unwanted play or backlash into the locking mechanism. An additional drawback to the washer design is that it is incapable of accepting flexible plungers because the angle generated by the washer is insufficient to prevent a flexible plunger from snaking out of the lock housing. The design therefore requires the plunger and the lock to be aligned on the same axis. When the lock is manufactured with less than perfect alignment in the socket the washer is subject to extraordinary wear causing the locking mechanism to fail prematurely.
A further drawback common to all the mechanical connectors is that they are necessarily manufactured with materials that can withstand the rigors of the locking mechanism. Commonly, the moving and contacting parts are fabricated from high carbon bearing grade steel and hardening the various parts, such as by heat treatment. Such steel bearing grade parts, however, are highly subject to corrosion, particularly when the prosthesis is used in environments offering exposure to liquids or other contaminants. Internal corrosion of the locking device can lead to seizure and failure of the moveable components.
Efforts to overcome the corrosion problem by forming the moving components of a corrosion resistant metal alloy, such as austenitic stainless steel (type 300 series) typically are not practical since such corrosion resistant steel cannot be hardened to the extent necessary for satisfactory life of the moving components. Alternately, forming the moving components of martensitic stainless steels, such as 440C, may provide a bearing grade steel which is hardenable. However, the corrosion resistance is less than that of the 300 series stainless steels. The result is that neither grade of stainless is suitable for the current lock designs. Therefore, amputees that live active lifestyles, who are prone to get their prosthesis wet are forced to live with locking mechanisms that corrode or fail to operate with peak efficiency.
The present invention addresses these potentially troublesome issues while combining the ease of use of an automatic lock with the stepless adjustability of the clutch and smooth pin lock.